The invention relates to a component for the transmission of light of high energy density with a wavelength between 250 nm and 400 nm, made of synthetic, high-purity fused vitreous silica, with a light input area having a light entry surface, a light output area having a light exit surface, and a light transmission portion disposed between the light input area and the light output area, and to the use of the component.
Such components are used for the transmission of ultraviolet light of high energy density, especially for the transmission of the light of excimer lasers, for the processing of materials, or in the medical field for the treatment of vascular diseases, or in ophthalmology. The use of such components for the transmission of laser radiation int he ultraviolet spectral range is limited, however, by what is known as "photodegradation." This refers to the reduction of transmission due to the attenuation induced by the high-energy radiation. This effect of "photodegradation," which is all the more pronounced the greater the energy density of the light to be transmitted is, has been observed also at the excimer laser wavelengths of 351 nm (Xf), 308 nm (XeCl) and 248 nm (KrF). In addition to plainly visible macroscopic defects, such as fusion of the surface, spalling or cracking, the reduction of transmission can be caused by microscopic faults in the glass structure.
In the paper written by Rod S. Taylor et al., "Dependence of the damage and transmission properties of fused silica fibers on the excimer laser wavelength," published in Applied Optics, Vol. 27, No. 15 (1988), the radiation resistance of components is studied in regard to the transmission of certain excimer laser radiation in the wavelength range of 193 nm to 351 nm. The components tested are fibers of undoped, synthetic fused vitreous silica with a hydroxyl ion content between 325 ppm and 1200 ppm. It is shown that, with the exception of the 351 nm excimer laser wavelength, the transmission of the fibers diminishes with increasing time of operation, but some time after irradiation by the high-energy laser light, a partial recovery of the transmission is to be observed. Furthermore, in this article, in a direct comparison of the changes in the transmission of fused silica fibers with a hydroxyl ion content of 400 ppm in comparison with those with 1200 ppm, it was found that, at the same wavelength and the same energy of the radiation transmitted by the fibers, the fibers with a hydroxyl ion content of 400 ppm underwent the least alteration of their transmission.
An article by C. Whitehurst et al., "Ultraviolet pulse transmission in optical fibers," Journal of Modern Optics, 1988, vol. 35, No. 3, 371-385, describes components which consist of a fused vitreous silica containing up to 1500 ppm of hydroxyl ions, and having a "destruction threshold" that tends toward the higher energy densities in comparison to pure, "dry" vitreous silica. In this article is described also the formation of components from fused vitreous silica containing hydroxyl ions, having a light input section with a light entry surface, a light output section with a light exit surface, and a light transmission section disposed between the light input section and the light output section, in which the light input section is in the form of a cone tapering in the direction in which the light enters.
From the paper, "Defects and Stress Phenomena in Optical Fibers," by H. Nishikawa et al., Proc. OFC, 1989, Paper THII, measurements of the absorption of optical fibers have become known, in which the fibers are characterized by their content of oxygen ions, the method of their manufacture, their chlorine ion content and their hydroxyl ion content. On the fibers with an under-stoichiometric content of oxygen an absorption was measured at 245 nm, and this absorption band is associated with a kind of defect intrinsically present on account of the under-stoichiometric oxygen content of the glass structure, namely so-called "oxygen vacancies." Similar measurements have also been published by R. Tohmon et al. in the paper, "Correlation of the 5.0- and 7.6-eV absorption bands in SiO.sub.2 with oxygen vacancy," Physical Review B, Vol. 39 (1989), No. 2. On the basis of measurements and corresponding calculations it is shown that the absorption bands at 5.0 eV (245 nm) observed in different high-purity fused silicas are to be attributed to oxygen vacancies and other oxygen-deficit defects. The fused silicas used for the measurements are also characterized by the nature of their manufacture and by their contents of chlorine ions and hydroxyl ions.
In connection with high-energy radiation in the wavelength range between 250 nm and 400 nm the known components of fused silica show a decrease of transmission from the start of the light input. Starting out from an initial value, the transmission of the components diminishes down to a "plateau" after which it changes only slightly even through fairly long operation. It has been found that these changes of the transmission from its initial level to the "plateau" definitely increase as the energy density of the radiation being transmitted increases. For a great number of applications, however, great changes in transmission during the use of the component are not tolerable. Since it is precisely the maximum transmittable energy density that is the deciding parameter for most applications, the usefulness of the known components is greatly restricted by the effect of this "photodegradation."